Corrosion resistant protective coating on aluminum and aluminum alloys



United States Patent 1 2,796,371 CORROSION RESISTANT PROTECTIVE COATINGON ALUMINUM AND ALUMINUM ALLOYS Charles W. Ostrander, Forest Hill, Md.,Nicholas R.

Congiundi, Decatur, Ala., and Walter E. Pocock, Baltimore, Md.,assignors to Allied Research Products, Inc.,

Baltimore, Md., a corporation of Maryland No Drawing. Application March16, 1955,

' Serial No. 494,812

20 Claims. (Cl. 1486.2)

This invention relates to the treatment of aluminum and alloys whereinaluminum is a principal constituent, to provide the same with acorrosion resistant coating. The coating formed is also one which formsan improved bond for organic finishes, such as paints, lacquers and thelike, and which may be suitably dyed when desired.

The invention comprises an aqueous solution of a chromic compoundselected from the group consisting of chromic acid and water-solublesalts thereof and ferricyanic or ferrocyanic acid and water-solublesalts thereof and mixtures of the same.

The attainment ofa corrosion resistant coating aluminum and alloysthereof is desirable in order to prolong the life of the object .beingcoated. Coatings utilized heretofore sutferedfromvarious disadvantageswhich the present coating composition is specifically designed toeliminate. f

For example, the aluminum protective coatings known heretofore generallyhave necessitated the use of electric current, heated or cooledsolutions, venting and the like, and the coating baths utilized toattainthe same necessitatevery long immersion timesin order to impart acoating of any practical use. 7 V Y Y.

The coating. Composition comprising the present development is designedto obviate these disadvantages. The coatings produced by use of ourcompositions exhibit greatly improved corrosion resistance and thecoating operation can be conducted with the solution at room temperatureas a simple dip treatment without need of venting. In addition, shorterimmersion times in the bath can be realized.

Further, as will be pointed out hereinafter, by carefully controllingthe operating conditions surrounding the utilization of our coatingbaths, we can realizecoloring in situ ranging from clear to yellow,brown and .blue. For example, by adding a ferric compound such as ferricchloride to the. basic composition'noted heretofore, a blue coatingresults with enhanced corrosion resistance as noted above;

In the case of yellow and brown coatings, for example, these can,if'desired, be bleached by dipping in hot water to a clear colorlessappearance; In addition,,they may be readily dyed, if required. 7

Accordingly, a principal object of this invention is the provision of anovel coating bath that will produce a coating on alurninumand'aluminumalloys of greatly enhanced corrosion resistance and farmore advantageous3 physical and operational characteristics, as recited above,

than did those previously utilized.

, Further objects include:

1. The provision of a novel coating composition which,

when dissolved in water, will yield'a coating bath that producesimproved corrosion resistant coatings on aluminum and alloys thereof,and canbe applied by dip, brush, or spray, as distinguished fromcomplicated electrolytic operations. n

2."A'fl.lrther object of this invention is'to provide a 'drypowdermixture for use in aqueous solutions to impart a corrosion resistantcoating to aluminum and alloys thereof, comprising a chromic compoundselected from the group s n' irt as' f eme g w r-nae ice salts thereofand ferricyanic or ferrocyanic acid and water-soluble salts thereof andmixtures of the same.

3. An additional object of this invention is to provide a compositionfor use in aqueous solutions to impart a corrosion resistant coating toaluminum and alloys thereof, comprising chromic acid and potassiumferricyanide wherein said chromic acid concentration is from .5 g./l. to5.0 g./l. and the potassium ferricyanide concentration can vary from .1g./l. to 50 g./l.

4. Still another objectof this invention is to provide a composition asdescribed heretofore including hydrofluoric acid and salts ofhydrofluoric acid, such as barium, sodium, potassium, and ammoniumfluoride, fiuosilicic acid and salts thereof, notably barium, sodium andpotassium fluosilicates, and fluoboric acid and salts thereof, such assodium, potassium and ammonium fluoborate, to increase the protectivevalue of the corrosion resistant coating, as well as to decrease thetime necessary for applications of such coating. Other soluble fluocompounds which will act in the same manner as those recited may beused.

5. An additional object of this invention is to provide the compositionof this invention including a fluo compound as described heretoforewherein, for example, the hydrofluoric acid concentration can vary from0.05 to 5. g./l., the fiuosilicic acid concentration from 0.075 to 8g./l., and the fluoboric acid concentration from 3.3 to 33 g./1.. Saltsof these three acids may be used in amounts chemically equivalent to thestated amounts of acid, in each instance.

6. 'Another and further object of this invention is to provide acompostion including a fluo compound listed heretofore wherein said fluocompound isof limited solubility thereby being maintained, throughself-replenishment, at a constant desired value, i. e., theconcentration of the semi-soluble fluo compound is maintained at aconstant desired value by virtue of the fact that the dipping'bath .issaturated with respect thereto.

Still-further objects and the entire scope of applicability of thepresent invention will become apparent from the detailed descriptiongiven hereinafter. It should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by Way of illustration only,since various changes andmodifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

vThese objects can be accomplished, according to the inventiom'by thedissolution in water of a composition including at least a chromiccompound selected from the group consisting of chromic acid andwater-soluble salts thereof and ferricyanic-or ferrocyanic acid andwatersoluble salts thereofand mixtures of the same. In addition, thiscomposition may include fluo compounds as recited above and a mineralacid, such as sulfuric, nitric or hydrochloric acid. These additionalcompounds are added in the proportions and under the conditions whichwill be hereinafter set forth in detail.

The composition of the present invention may also be supplied to thecoating plant as a dry mixture and made up into a suitable bath by theaddition of water, or it may be supplied to the coating plant as aconcentrated aqueous solution which may be diluted by the consumer toprovide the necessary coating bath.

We have discovered that it is possible to eifectively protect aluminumsurfaces by a dip, brush or spray with a solution containing compoundsas heretofore set forth. Not only is an improved protective coatingdeposited on the aluminum surface, but such coating has excellentbonding power for subsequent organic finishing, e. g., with paints andlacquers, and, in some cases, where desired, can be dyed readily withvarious colors. .In addition, the coating operation can be modified toproduce, in situ, protective coatings of many and varied desired colorsby reason of the composition of the coating bath and the operationalprocedure used.

The aluminum or aluminum alloy articles to be coated are firstpreferably cleaned by conventional methods which remove organic matterand metallic oxides. The articles are then subjected to a dipping,brushing or spraying step in an aqueous solution comprising thecompositions of our invention.

As heretofore noted, the coatings realized by utilization of ourinvention exhibit greatly enhanced corrosion resistant characteristics.It is believed that when an aluminum or aluminum alloy surface issubjected to solutions of the character comprehended by this invention,a chemical reaction takes place between the particular solution and thealuminum surface. The nature of this chemical reaction cannotspecifically be described but the final coating, after drying, istightly adherent to the aluminum surface and, as a matter of fact,appears to be integral therewith.

A more complete understanding of the procedure of this invention may behad by reference to the following illustrative examples of actualoperations in accordance with the invention.

EXAMPLE I Chromic acid g./l 25 Potassium ferricyanide g./l Temperature F70 pH 0.8 Immersion time minutes 5 The above composition is considered atypical formulation of chemical ingredients which, when made into anaqueous solution, produced protective coatings as heretofore described.Generally, the concentration of the ingredients can be varied to from .5g./l. to 50 g./l. of chromic acid and from .1 g./l. to 50 g./l.potassium ferricyanide. In addition, other chromic compounds, such aspotassium or sodium bi-chromate, can be substituted for the chromicacid. The operating temperature in Example I can be between 32 to 160 P.which constitutes the temperature range generally contemplated in ourinvention. In addition, an immersion time of from five seconds to tenminutes has been found to be the preferred time rangewithin which thecompositions of our invention is operable. The exact immersion time isdependent upon the degree of corrosion resistance desired and, further,whether it is desired to produce a color in situ rather than to utilizea later dyeing step.

With respect to this latter point, it should be noted that a clearcoating will be obtained by the utilization of the composition ofExample I if an immersion time of from one to three minutes is used. Ayellow coating can be realized with a three to ten minute immersiontime.

The pH may be varied within a range of about 0.1 to 2.5 measured by anelectrometric pH meter, and this pH variation may also be utilized tocontrol color.

A salt spray test on 24 S. T. aluminum (an aluminum alloy containing upto about 4 /2% copper) having a yellow coating imparted by thecomposition of Example I exhibited less than 1% white corrosion after766 hours exposure.

By the addition of a fluo compound, such as a salt of fluosilicic acidto the formulation of Example I, equivalent or better coatings can beobtained with much shorter immersion times. An example of this modifieddipping bath follows:

EXAMPLE II Chromic acid g./l 50 Potassium ferricyanide g./l 5 Sodiumfluosilicate g./l 5 Temperature F 70 Immersion time seconds 60 pH 0.4

As previously indicated, the general temperature range of 32 to 160 F.is applicable to the above composition. A temperature range of 70 F. to90 F. is preferred. We should state that the immersion time canpreferably vary from five seconds to about five minutes or over,depending upon the color or thickness of coating desired to be attained.It should be noted that, upon the addition of the fluosilicate, heaviercoatings can be realized with shorter immersion times.

Using the formulation of Example II under the conditions described, acoating was produced which, on 24 S. T. aluminum, showed less than 1%white corrosion after 1000 hours in a salt spray. It can be appreciatedthat the sodium fiuosilicate significantly decreased the immersion timerequired to produce a coating possessing corrosion resistancesubstantially equivalent or better to that obtained by utilization ofthe basic formulation of Example I. This effect of the sodiumfluosilicate can best be explained by stating that this compoundaccelerates film formation.

As we stated heretofore, the color imparted by the formulations ofeither Example I or II or subsequent examples may be varied bycontrolling the immersion time. However, a variety of colored finishescan also be obtained ranging from clear through yellow to dark brown byadjusting the pH of the dipping solutions between below pH 0.1 and pH3.0. Any of the known pH controlling agents can be used in Examples Iand II.

Examples of the use of various chromic, ferricyanide,

ferrocyanide and fluo compounds are hereinafter set forth.

EXAMPLE III Chromic acid g./l 5 Potassium ferricyanide g./l 2.5 Sodiumferrocyanide g./l 2.5 Sodium fluoborate g./l 5 pH 1.5 Immersion timeminutes 5 Temperature F 70 The above formulation yielded a yellow-browncoating which, on 24 S. T. aluminum, showed less than 1% corrosion after1000 hours in salt spray.

EXAMPLE IV Sodium bichromate g./l 7.5 Potassium ferricyanide g./l 5Sodium fluoride g./l 1 Nitric acid (42 B.) ml./l 3 pH 1.5 Immersion timeminutes 5 Temperature F The above formulation yielded results similar tothose described heretofore and is of particular interest since itillustrates the use of a mineral acid to adjust the pH of the bath.

EXAMPLE V To the formulation of Example I can be added 1 to 10 ml. ofsulfuric acid. We added 8 ml. of sulfuric acid to lower the pH to about1.2 and the coating that resulted had a dark clear reflective colorunder what were otherwise the same operating conditions of Example I.

As noted heretofore, the concentration of the basic formulation,typified by chromic acid and potassium ferricyanide, varied from .1g./l. and .5 g./l. respectively to 50 g./l. for both of these compounds.Different concentration ranges would be applicable to the use of saltsof chromic acid, or ferricyanic or ferrocyanic acid or salts thereof aswell as mixtures of the same. These different concentration ranges canbe calculated using the .1 g./l. and .5 g./l. respectively to 50 g./l.ranges noted heretofore. In other words, the calculations should be suchthat the same amount of hexavalent chromium and ferricyanide orferrocyanide is realized in the mixture being formed. On this latter,point, it, should be noted that the ferricyanic and ferrocyanic radicalsdo not differ in struc: ture but merely in valence. I g

The same type of calculation can be utilized to determine theconcentration rangeof the various fiuo compounds, that can besubstituted for the fluo acids whose concentration ranges were notedheretofore. For example, the fluosilicic acid, range was given as from0.075 to 8 g./l. from such figures, the sodium fluosilicateconcentration range, if such salt was substituted for fluosilicic acid,is calculated at about 0.1 to 10 g./l.

As previously indicated, the desired chromic compound can be obtainedfrom a number of compounds. For example, chromic acid, sodium orpotassium bichromate, or sodium or potassium chromate canbe utilized.Chromic acid or sodium or potassium bichromate'is preferred foreconomical reasons.

EXAMPLE VI Chromic acid g./l. Potassium ferricyanide 5 g./l. Bariumfluosilicate 2.0 g./l. Temperature Room temperature (80 F.). pH 1.5. 1

A yellow corrosion resistant coating was produced by utilizing the aboveformulation.

This formulation illustrates the use of a fluo compound of limitedsolubility in order to impart a partial self-replenishment to thedipping bath. Barium fluosilicate is of such solubility in water as toprovide a solution of fixed concentration with respect to the fluocompound, i. e., the excess undissolved barium fluosilicateautomatically maintains the fluo concentration at a constant desiredvalue. Other examples of such a feature would include the use ofpotassium fluosilicate, barium fluoride, aluminum fluoride or magnesiumfluoride.

EXAMPLE VII Chromic Acid 5 g./l. Sodium ferrocyanide 1.5 g./l. Bariumfluosilicate 2.0 g./l. pH 1.5.

Operating temperature Room or above. Immersion time 5 minutes.

EXAMPLE VIII Chromic acid 5 g./l. Ammonium ferrocyanide 1.0 g./l. Bariumfluosilicate 2.0 g./l.

Operating temperature Room or above. Immersion time 5 minutes.

EXAMPLE IXDry Powder Mix Percent by weight Chromic acid 54 Potassiumferricyanide 11 Barium nitrate 20.5

Sodium fluosilicate 14.5

EXAMPLE X Chromic acid 5 g./l.

Potassium ferricyanide 1 g./l.

Barium nitrate 1.9 g./l.

Sodium fluosilicate 1.35 g./l.

Operating temperature Room temperature (about 80 F.).

Immersion time About /2 minute.

In this formulation the use of barium nitrate and sodium fluosilicate inaqueous solution results efiectively in the formation of bariumfluosilicate and has the same effect as if barium fluosilicate alonewere used. The choice of ingredients in this caseis made for practicaleconomical reasons.

The temperature may vary between 32 and 160 F., the preferred rangebeing 65 to F. The time may vary between 1 second to ten minutes, thepreferred range being about /2 minute to six minutes.

It can be appreciated that Examples VI-X represent formulations thatexhibit and produce preferred effects. In other words, the basic chromicand ferrior ferrocyanide compositions are enhanced by the fluosilicatecompound addition in that immersion time is decreased and a heavier,more even coating is produced. Add to this the fact that aself-replenishing feature is imparted by the barium fluosilicate, asheretofore set forth, and the fact that a preferred overall dippingcomposition is realized has been made evident.

Referring to Example IX, from about .1 ounce to 5 ounces of the powdermix of Example IX per gallon of water are useful as in Example X.

Conclusions The present invention provides a new coating composition foruse in aqueous solutions to impart a corrosion resistant coating toaluminum and alloys thereof. The new composition provides bettercorrosion resistance, an excellent bond for paint and varnishes, aclearer coating and shorter immersion times at room temperature (80 F.).In addition, by varying the ingredients and controlling the pH of thedipping solution and immersion times of the articles being coated,various colors in situ can be realized.

The novel principles of this invention are broader than the specificembodiments recited above, and rather than unduly extend this disclosureby attempting to list all the numerous modifications which have beenconceived and reduced to practice during the course of this development,these novel features are substantially defined in the following claims.

This application is a continuation-in-part of our copending applicationSerial No. 400,774 filed December 28, 1953.

We claim:

1. A composition for use in aqueous solution to impart acorrosion-resistant coating to aluminum and alloys thereof in which thecoating-producing ingredients consist essentially of hexavalentchromium, a fluorine-bearing compound, and a cyanide selected from thegroup consisting of ferricyanic acid, feirocyanic acid, and saltsthereof and mixtures of these acids and salts, and a water solublesource of barium.

2. A composition according to claim 1 wherein the fluorine-bearingcompound is a fluosilicate.

3. A composition according to claim 1 wherein the source of barium isbarium fluosilicate.

4. A composition according to claim 1 wherein the fluorine-bearingcompound is a fluosilicate and the source of barium is barium nitrate.

5. A composition according to claim 1 wherein the source of barium isbarium fluoride.

6. An aqueous solution as claimed in claim 5 including a mineral acidand having pH range from about 0.1 to pH 3.0.

7. An aqueous solution to impart a corrosion-resistant coating toaluminum and alloys thereof in which the coating-producing ingredientsconsist essentially of water, hexavalent chromium, a fluorine-bearingcompound, and a cyanide selected from the group consisting offerricyanic acid, ferrocyanic acid, and salts thereof and mixtures ofthese acids and salts, and a. water soluble source of barium.

8. An aqueous solution according to claim 7 wherein the fluorine-bearingcompound is a fluosilicate.

9. An aqueous solution according to claim 7 wherein the source of bariumis barium fluosilicate.

10. An aqueous solution according to claim 7 wherein the source ofbarium is barium nitrate.

11. A method as claimed in claim 10 wherein the pH of the solution isfrom about pH 0.1 to 3.0.

12. A method of imparting a corrosion-resistant coating to aluminum andalloys thereof which comprises subjecting the latter to an aqueousacidic solution comprising water, hexavalent chromium, afluorine-bearing compound, and a cyanide selected from the groupconsisting of ferricyanic acid, ferrocyanic acid, and salts thereof andmixtures of these acids and salts, and a water soluble source of barium.

13. A method according to claim 12 wherein the fluorine-bearing compoundis a fluosilicate.

14. A method according to claim 12 wherein the source of barium isbarium nitrate.

15. A composition consisting essentially of chromic acid, potassiumferricyanide, barium nitrate and sodium fiuosilicate.

16. A composition consisting essentially of chromic acid, potassiumferricyanide and barium fiuosilicate.

17. A composition consisting essentially of chromic acid, sodiumferrocyanide and barium fiuosilicate.

18. A composition consisting essentially of chromic acid, ammoniumferrocyanide and barium fiuosilicate.

19. A composition for use in aqueous solution to impart acorrosion-resistant coating to aluminum and alloys thereof consistingessentially of chromic acid 5 g./l., potassium ferricyanide 1 g./l.,barium nitrate 1.9 g./l. and sodium fluosilicate 1.35 g./l. saidcomposition being in water solution.

20. A dry powder mix for use in aqueous solution to impart acorrosion-resistant coating to aluminum and alloys thereof consistingessentially of chromic acid 54% by weight, potassium ferricyanide 11% byweight, barium nitrate 20.5% by weight, and'sodium fluosilicate 14.5% byweight, said mix being used in amount of about .1 ounce to 5 ounces pergallon of water.

References Cited in the file of this patent UNITED STATES PATENTS2,114,151 Romig Apr. 12, 1938 2,276,353 Thompson Mar. 17, 19-422,453,764 Snyder Nov. 16, 1948 FOREIGN PATENTS 1,085,484 France July 28,1954

1. A COMPOSITION FOR USE IN AQUEOUS SOLUTION TO IMPART ACORROSION-RESISTANT COATING TO ALUMINUM AND ALLOYS THEREOF IN WHICH THECOATING-PRODUCING INGREDIENTS CONSIST ESSENTIALLY OF HEXAVALENTCHROMIUM, A FLUORINE-BEARING COMPOUND, AND A CYANIDE SELECTED FROM THEGROUP CONSISTING OF FERRICYANIC ACID, FERROCYANIC ACID, AND SALTSTHEREOF AND MIXTURES OF THESE ACIDS AND SALTS, AND A WATER SOLUBLESOURCE OF BARIUM.